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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

OXIDISING

295

sulphide, filtered, boiled to expel excess of hydrogen sulphide, made alkaline
with ammonia, heated and filtered. In the nitrate, acidified with acetic
acid, the zinc is tested for with potassium ferrocyanide.

OXIDISING

For increased protection against atmospheric agencies or for the sake
of appearance, many objects are coated either chemically or mechanically
with a thin layer of oxide, which imparts to them a brown or bluish-brown
coloration.

Similar coloration may, however, arise spontaneously during the working
of the obj ects owing to reheating and it is not always easy to decide if the
oxidation is artificial or spontaneous. One distinguishing character is the
regularity and uniformity of the layer of oxide with artificially oxidised
objects, in comparison with the irregularity of layers of oxide formed spon-
taneously ; the following tests are based on this criterion.

1.  For Copper and Brass Objects.—The surface of the object is
thoroughly freed from grease by means of benzene and treated with a drop
of 5% mercuric chloride solution.    If the layer of oxide is very regular
and compact, the reagent will not get into contact with the metal and no
reaction will be observed.    If, however, the oxidation is irregular, the mer-
curic chloride undergoes reduction at the surface of the metal, forming a
grey spot.

2.  For Obj ects of Iron.—After being cleaned with benzene, the oxidised
surface is moistened with a drop of 5-6% copper sulphate solution.   If
the oxidation is irregular and hence not artificially formed, a spot of metallic
copper appears either immediately or after some time, whereas, if the oxida-
tion is regular and uniform, any reduction which may occur will be observable
only after the lapse of a long period.

With reference to this test it is, however, to be noted that sheet metal
(lamiere bleu-lisse) is now put on the market covered with a regular layer
of ferroso-ferric oxide of a bluish colour, which, although obtained during
the process of rolling is very regular and uniform and does not allow of any
reduction of copper sulphate.

Such sheet is nevertheless readily distinguishable from that oxidised
by means of reagents or the like.

1.  Technical test.   When a sheet of this character is bent at right angles
it sheds its oxide at the bend in the form of scale and shows the naked metal,
whereas sheet oxidised by reagents exhibits little change.

2.  Microscopic test.    Further, microscopic examination, in reflected light
and under a magnification of 100-120 diameters, of the surface of such sheet
reveals numerous minute fissures which have been made in the oxide during
rolling and lie parallel to the axis of the rolls.   Figures 28, 29 and 30 [see
plate) represent reproductions of microphotographs of the surfaces of three
types of such sheets from Germany, England and Belgium, while Fig. 31
shows the appearance of a Belgian sheet at the bend.

Metal which has been oxidised with reagents exhibits no such parallelion with hydrochloric acid, pre-